1. Field of the Invention
The present invention relates to a process and an apparatus for “flow soldering” (which is also referred to as “wave soldering”) wherein electronic components are mounted onto (or bonded to) a board (or a substrate) by means of a lead-free solder material.
2. Description of Related Art
In recent years, it has been strongly desired to increase the reliability of an electronic circuit board which is contained in a downsized electronic device while still maintaining and a high performance of the electronic device. Therefore, there is an increasing demand to increase a reliability property such as the thermal shock resistance and the mechanical strength of a connecting portion which is formed by soldering an electronic component onto a board in the field of mounting the electronic components.
Moreover, while concern about the protection of global environment is increased in a worldwide scale, a regulation or legal system to control industrial waste treatments is being arranged. Although an Sn—Pb based solder material, which contains Sn and Pb as its main components (e.g. a so-called “63 Sn-37 Pb” eutectic solder material), is generally used in an electronic circuit board which is incorporated in an electronic device, lead contained in such solder material may cause environmental pollution if it is subjected to an inadequate waste treatment. As a result, research and developments are carried out as to a solder material which does not contain lead (i.e. a so-called lead-free solder material) as an alternative to a solder material which does contain lead.
A conventional flow soldering process for producing an electronic circuit board by connecting a electronic components to a board such as a printed board as well as an apparatus for such process will be described with reference to drawings below. FIG. 3 shows a schematic view of the conventional flow soldering apparatus.
At first, a board is prepared prior to soldering, wherein through holes are formed through the board, and an electronic component is located thereon by inserting a lead (e.g. an electrode) of the component into the through hole from an upper surface of the board. In such a board, a land which is made of copper or the like is formed on a region (A+B+C, see FIG. 4) consisting of a surface (A) which defines the through hole as well as an upper surface portion (B) and a lower surface portion (C) of the board, where portions (B) and (C) surround the through hole, and such a land is connected to a circuit pattern on the upper surface of the board. On the other hand, regions of the upper surface and the lower surface of the board, except for the lands, are covered by a solder resist.
Next, the board is subjected to a pre-treatment in which the lower surface of the board on which no electronic component is located thereon is applied with flux by means of a spray fluxer (not shown). The pre-treatment is conducted in order to improve wetting and spreading of the solder material on a surface of the land by removing an oxide film (such as a film formed by natural oxidation) which is inevitably formed on the land.
Then, referring to FIG. 3, a thus prepared board (not shown) is put into the flow soldering apparatus 60 while the upper surface on which the electronic component is located is kept upward (with regard to the drawing), and the board is mechanically transferred in a direction of the arrow 61 inside the flow soldering apparatus 60 with a substantially constant velocity by means of a conveyer. In the flow soldering apparatus 60, the board is first heated in a preheating zone by means of a preheating unit (or preheater) 62 in order to make the flux applied to the board, according to the pre-treatment, effectively display its activity ability.
Thereafter, when the board is conveyed into a solder material supplying zone located above solder wave nozzles 64 and 65, the solder material (not shown), which is in a molten state by heating beforehand in a solder material supplying unit 63, is supplied to the board from its lower side through the primary wave nozzle 64 and the secondary wave nozzle 65 in the form of a primary wave and a secondary wave respectively. The solder material thus supplied goes up from the lower surface of the board by means of the capillary action through an annular space between the surface of the through hole (i.e. the land) and the lead which is inserted through the through hole from the upper surface of the board. Thereafter, the solder material naturally cools by releasing its heat to surrounding areas of the board with its natural cooling rate, so that the solder material thereby solidifies to form a connecting portion of the solder material (or a so-called “fillet”). In this step of supplying the solder material (or the step of flow soldering), the primary wave functions so as to sufficiently wet the surfaces of the lead and the land with the solder material, and the secondary wave functions so as to remove the solder material on regions covered with a solder resist. As a result, the solder material does not form a bridge by remaining on the board and thereby solidifying between the lands (the bridge is not desirable because it causes a short circuit), and the solder material does not form a cornute projection, thereby controlling (or conditioning) the form of the fillet.
As described above, the fillet (or the connecting portion) made of the solder material is formed to electrically and physically (or mechanically) connect the lead of the electronic component and the land formed in the board.
The fillet made of the solder material as described above is required to have a sufficiently large connecting strength between the lead of the electronic component and the land of the board in order to provide a high reliability of the electronic circuit board. However, referring to FIG. 4, if the electronic circuit board 70 is produced by using the lead-free solder material according to the conventional flow soldering process as described above, the solder material having been wetted and spread on the surface of the land 73 (which is located to cover an inside surface which defines the through hole 72 perforated through the board 71 as well as regions which surround the through hole 72 on the upper side and the lower side of the board 71) partially peels off at an interface between the solder material and the land as indicated by the arrow 80 upon the solidification of the solder material. There thus arises a problem in that the connection between the land 73 and the fillet 74 made of the solder material becomes insufficient and thereby a high connecting strength between the lead 75 and the land 73 cannot be obtained.
Such a phenomenon of the peeling-off of the fillet 74 from the land 73 is generally referred to as a “lift-off” phenomenon, which frequently occurs when the lead-free solder material is used, although it scarcely occurs when the Sn—Pb based solder material is used. The lift-off phenomenon notably occurs especially in the cases where the lead-free solder material which contains Sn and Bi (such as an Sn—Ag—Bi based material) is used and in the cases where the lead-free solder material for connecting a lead which is plated with an Sn—Pb based material is used.
As a reason for the occurrence of the lift-off, it could be generally considered that the solder material used for the flow soldering and/or a metal material which can elute into the solder material upon soldering (e.g. a plating metal for the lead) forms a weak alloy having a lower melting point than that of the initial solder material and a composition which is different from that of the initial solder material (hereinafter, such an alloy is merely referred to as a “low-m.p. alloy”) upon the solidification of the solder material from its molten state.
When the molten solder material is at a high temperature it loses its heat mainly via the lead 75 which comes from the electronic component (not shown). In the solder material (the fillet) 74 which is supplied and which adheres to the board 7 1, a temperature gradient is formed by a flux of such heat passing through the lead 75. The solidification of the solder material 74 progresses according to its temperature gradient, wherein the lowest temperature portion of the solder material 74 is formed at the top of the solder material 74, which is indicated by the arrow 81, and the highest temperature portion is formed in the vicinity of the interface between the fillet 74 and the land 73, which is a good heat conductor. As a result, the solidification begins at the top of the solder material 74 and ends at the interface between the land 73 and the fillet 74 in due course. Upon solidification, the low-m.p. alloy as described above is distributed (or segregated) in a greater amount in a still molten portion of the solder material which has not yet solidified, such that the low-m.p. alloy is transferred to and concentrated (or segregated) in the molten portion in relation to the progress of the solidification. That is, a segregation phenomenon occurs upon solidification of the solder material, and as a result, the low-m.p. alloy 76 gathers at the interface between the land 73 and the fillet 74 which is where the solidification occurs last. In accordance with this manner of solidification, the fillet 74 solidifies at the top first and attaches to the lead 75. As a result, a tension is generated in the direction of the arrow 82 by a contraction due to the solidification of the solder material, and a tension in the direction of the arrow 83 is generated by the thermal contraction of the board 71. It could be considered that the weak low-m.p. alloy 76 which is enriched in the vicinity of the interface between the land 73 and the fillet 74 as described above can not endure these tensions, and as a result, the peel-off phenomenon is caused at the interface as shown in the direction of the arrow 80.